In a number of applications, various users of electronic systems have found it desirable to interact with the electronic systems through a display, e.g., a computer monitor, a television, a personal digital assistant (PDA) and automated teller, in order to input information into the system. Thus, a number of designers have utilized various technologies in an attempt to improve touch sensitive input devices, such as touch sensitive screens and digitizer pads. Various types of these input devices have been designed using, for example, capacitive, resistive, infrared, surface acoustic wave (SAW) and guided acoustic wave (GAW) technologies.
Most touch sensitive screens implementing capacitive technology have been realized by fusing a transparent thin film conductive coating onto a glass surface. A low voltage AC field has then been applied to and distributed across the conductive coating such that when a user's finger made contact with a surface of the screen it capacitively coupled with the AC field drawing a small amount of current to the point of contact. In such screens, the current flow from each corner of the conductive coating is proportional to the distance to the user's finger and the ratios of the current flows are measured by a control unit to determine where the user touched the screen.
Typically, resistive touch sensitive screens have utilized a contacting member, e.g., a stylus, to form a momentary connection between two semi-flexible conductive layers. A control unit then determines where the contacting member touched the screen by sensing a change in voltage. Infrared technologies have implemented control units that detect a change in infrared light propagation, initiated when a user touches a touch screen, to determine where the user touched the screen.
Touch pads implementing SAW technology have generally included a glass panel with transducers that transmit and receive surface waves over the face of the touch pad. When a finger or other object touched the surface of the screen, a portion of the energy of the wave was absorbed at that location, which could then be determined by a control unit, based upon the presence of interference patterns in the acoustic wave. Typical characteristics of input devices implementing the above-referenced technologies are set forth below in Table 1.
TABLE 1CAPACITIVERESISTIVEINFRAREDGAWSAWResolution (PPI)>250>200815033Z-axis?NoNoNoYesYesAmbient LightUnaffectedUnaffectedVariesUnaffectedUnaffectedActivationTactileTactileProximityTactileTactileParallax?NoNoYesNoNoResponse Time 5-15 ms 5-10 ms18-40 ms18-50 ms20-50 msTransmissivity85-92%65-80%100%92%92%Sensor Reliability20 M touches/point35 M touches/point138 K hrs MTBFUnlimited50 M touches/pointIntegrationInvasive or non-Invasive. OpticalInvasive or non-Invasive. OpticalInvasive. Opticalinvasive.bonding required.invasive.bonding required.bonding required.Stylus TypeRequires conductiveNo stylus limitation.No stylus limitation.Requires soft,Requires soft.stylus. Cannotenergy absorbingenergy absorbingdetect gloved finger.stylus.stylus.Sensor DriftSubject to drift.Subject to drift.Not subject to drift.Not subject to drift.Not subject to drift.Requires repetitiveRequires repetitivecalibration,calibration.DurabilityConductive layerSensor isNot susceptible toDifficult to scratch.Difficult to scratch.subject to wear.susceptible toscratching, noGlass overlay isGlass overlay isscratches andoverlay, solid state.breakable.breakable.abrasions.Dust/DirtAccumulationNot affected by dustWill operate withNot affected by dustWill operate withResistanceaffects performance.and dirt.dust and dirt.and dirt.dust and dirt.ExcessiveExcessiveaccumulation mayaccumulation mayaffect performance.affect performance.
The various technologies have relative advantages and disadvantages depending upon the specific application. None of the currently available technologies are generally suitable for automotive display applications, which require minimal interaction time between driver and touch sensitive input device, allowing the driver to keep his/her eyes on the road and drive in a safe manner. Further, in general, most automotive display applications require only a limited number of touch sensitive “spots,” as opposed to devices such as personal digital assistants that allow high resolution touch sensitive response. In addition, the driver may be wearing gloves, which affects the ability of currently available touch screens to properly resolve a point of contact on a display's surface.
What is needed for automotive applications is a transparent overlay input device that is durable and relatively inexpensive to manufacture. Ideally, such a device could be added to existing display systems. It would also be desirable if the transparent overlay input device minimized electromagnetic interference (EMI), so as to not adversely affect other electronic systems of the motor vehicle.